The present invention relates to semiconductors and, in particular, to metal contacts formed on semiconductors to improve the barrier energy range.
It is known that a potential barrier forms when a metal and a semiconductor are brought into contact and, further, that the barrier height is dependent upon the electronegativity of the metal (C. A. Mead, Solid State Electron. 9,1023-1966). When the barrier is small, the contact is said to be `Ohmic`. When large, (&gt;&gt;kT), it is `rectifying`. Ohmic contacts to p-type wide-band-gap semiconductors require highly electronegative metals, especially if heavy doping is not possible (C. A. Mead, Ohmic Contacts to Semiconductors, edited by B. Schwartz -- Electrochem. Soc., NY, 1969, p 6). On a n-type semiconductors barrier height increases with increasing metal electronegativity. Such barriers are useful for transistors and solar cells as well as for materials which cannot easily be made both p-type and n-type - See: Sze, Physics of Semiconductor Devices, Wiley, NY, 1969, pp 410-412, 669; Stirn and Yeh, Appli, Physics Lett. 27, 195, 1975; Haeri and Rhoderick, Proceedings of Conference on Metal Semiconductor Contacts, Manchester -- The Instit. of Physics, London, 1974, p 84.
Over 10 years ago, C. A. Mead and co-workers (Solid State Electron, 1966 -- supra) demonstrated that on many semiconductors the barrier heights of the common metals can be ranked or ordered by the so-called Peter Pauling electronegativity scale. (Linus & Pauling, Chemistry -- Freeman, San Francisco, 1975, p 174.) In the present drawings, FIG. 1 demonstrates this scale applied to a number of representative elements. To the right of the scale are the very electronegative elements which, as stated, produce high barriers to n-type semiconductors. The commonly-used elemental metals such as Al, In, Pt and Au occupy a rather short range of less than 1 or, as shown, a range that extends from 1.5 (Al) to 2.4 (Au). Actually, there are a number of more electropositive metals which produce lower barriers to n-type semiconductors than the commonly-used metals, but, unfortunately, these metals are very reactive.
Of all the elemental metals, Au, the most electronegative metal, produces the highest barrier to n-type semiconductors and, for this reason, gold contacts have been rather commonly used. Nevertheless, in many situations, contacts that are even more electronegative than gold are desired both for the higher barriers to n-type and the ohmics to the p-type semiconductors. However, there are only eight elements more electronegative than Au and not one of the eight is a conductor. To obtain increased electronegativity there is a need to consider compounds rather than the elemental metals. It also is essential that the compound be a metallic conductor and, in this regard, it is to be noted that a number of highly electronegative elements to the right of Au are insulators. For example, both sulfur and nitrogen are very electronegative insulators.
As should be apparent in the foregoing discussion, a principal object of the invention is to produce a more electronegative contact than Au for n-type semiconductors.
Another object is to produce smaller barriers or ohmics to p-type semiconductors.
As will be described, the objects are achieved by the use of a new contact material, polymeric sulfur nitride, (SN).sub.x.